Determination
by Insanity Allegra
Summary: Two souls, one body. One pacifist, one genocidal. Bill had prepared himself to take on one of them, the silly little girl called Frisk, but he certainly didn't plan for the appearance of Chara... Then again, if he hadn't crossed the line, Chara wouldn't be so determined to bring him down, huh? Aftertale Underfalls AU, where the gang moved to GF after the True Pacifist ending.


I do not own Undertale, Toby Fox does. I do not own Gravity Falls, Disney does.

~! #$%^&*()_+

"Come on, little girl, fight back!" The triangle demon roared.

She shook her head, shoulder-length brown hair flopping in the wind. **Stand strong. Be determined. We can win this. I believe in you, Frisk!**

Dipper and Mabel were out of commission. Early into the confrontation, the Mystery Twins had been slammed into the side of the Shack. They were out cold. Or at least Frisk _hoped_ they were. **Don't think about it.**

Toriel, Papyrus, Undyne, Flowey, Alphys, and Sans were nowhere to be seen, and she worried for them.

The sky was a sickly yellow-orange, the grass dead and crispy under her sneakers. The air tasted foul, fouler than even the Hotlands' barren air had. The world was ending, just as things had become peaceful between humans and monsters, literally _months_ after they had found a loving, wonderful family that didn't care that she wasn't only her but also him – **They welcomed us with open arms, despite us being two instead of one.**

We can do this.

 **We must do this.**

Peacefully.

 **I suppose.**

"It doesn't have to be this way, Bill!" She shouted over the roaring wind. Dead tree branches whipped about like they were trying out for the part of the Whomping Willow in the Harry Potter films. "You can still stop this! You can make a better future..."

"SILENCE! You're a fool, little girl! 'The Angel of Monsters' won't even lift a finger to stop the end of all life! Pathetic!" The demon pointed at her, and dozens of energy lances shot for her.

Well, she'd learned to dodge from the best of them. Left right, right, right, up, back, spiral... She was practically on autopilot, mind working furiously as she desperately tried to think up a way to end this... One of the lances clipped her on the side, and she stumbled, falling to the dirt.

"Bill, we don't have to fight! I don't want to fight you!"

"TOO BAD, LITTLE HUMAN!" The demon built up a massive lance, and Frisk panicked, attempting to scramble to her feet. **Come on, Crybaby, get up! I can't exactly help you, and if you die, we're both goners!** She managed to tumble out of the way of the blast, feeling the shock wave tear at her shirt. Holy shit that was close!

Frisk got shakily to her feet. "Please, Bill. I _won't_ fight!"

"You're i _n my way_ , little girl!" He nearly shrieked, starting up more lances. **Fuckfuckfuckfuck...**

And then a large, looming shadow was between them and the demon.

"If you don't get away from my girl, you're gonna have a _bad_ time."

Sans! **Sans!**

Frisk hid behind the skeleton, feeling safe and relieved because Sans was one of the strongest monsters in the Underground. He'd protect her to the end... Just like a father should.

She didn't look at the battle, but that didn't mean she couldn't see it. He watched intently, curious and proud and determined. And so she saw what he did. It was epic, and both parties were tough and skilled... But in the end, _somebody_ had to win.

But Frisk was _not_ expecting it to be the demon. The energy lance struck true, straight through Sans' spine, going all the way and continuing towards Frisk despite already having struck down its target but she was in too much shock to dodge or block or think or breathe, her eyes and his eyes locked on their father-figure as he started to turn around, his last gasp going towards stopping the deadly spear from impacting his little girl. He landed on top of it, pinning it to the ground, and reached a shaking arm towards Frisk.

"D-daddy?"

"I-" he coughed. "I love ya, kiddos... Take good care-a my bro, kay? He's such a cinnamon roll, y'know...?"

 **S-Sans?**

"Daddy! Daddy don't go!" Tears blurred Frisk's vision as she scrambled over, taking the bony hand in her own small one.

"Sorry, kiddos... G-Guess I couldn't protect you..." He started to rapidly dissolve into dust, and Frisk began to sob in earnest. Soon, all that was left of Sans was his big, fluffly coat.

" _Daddy_..." She looked up at Bill with streaming eyes. "Y-You... You k-killed Daddy... YOU KILLED MY DADDY!"

Bill only laughed. "And doesn't that make you angry? Don't you want to hurt me, don't you want to fight back?"

 **She does not. You will _not_ taint her soul today, demon.**

"... Who said that?" Bill demanded, looking around.

Frisk scrubbed frantically at her eyes, turning to him.

He'd been with her for almost as long as she could remember, but nobody else had ever seen him, or heard him. And it wasn't like he'd been silent during this particular fight. "C-Chara?"

 **I won't let him hurt you anymore.** The half-transparent (to Frisk's eyes) preteen growled. **I won't let him win. The sight of you, lying there brokenhearted...** His eyes flashed, glowing a deep, pulsing red. **It fills me with** _ **determination**_ **.**

Even as Frisk watched, her companion seemed to become more solid, more _real_.

 **William Cipher. Prepare to die.** A flash of light heralded the appearance of a worn old knife in his hand. **You killed my father. You hurt my sister. But worst of all, you made her cry. And** _ **I**_ **am the only one who gets to make Frisk cry!** He lunged for the stunned demon, who was very much _not_ used to random souls suddenly becoming corporeal just to attack him, and didn't shield in time. The knife struck him directly in the eye, and something in his body and magic _gave_ , and he cracked apart, falling into tiny shards as he let out an unearthly scream.

All at once, the apocalypse stopped, life returning to the formerly dying world, the trees regaining their leaves and needles, the grass becoming lush and green again, the sky and air becoming blue and fresh. But though both Frisk and Chara looked hopefully towards the big blue coat on the floor, Sans did not revive.

Frisk began to cry again. Her companion could feel his body beginning to fade away again, so he pushed towards the winter warmer, gathering it up in arms that were becoming increasingly intangible, and bringing it over to drape over Frisk's shoulders. **Shhh, Crybaby... I'm here... I ain't goin' nowhere...** He hugged her. **I dunno how it happened, sister, but I love you. And I can't promise it'll be okay. But I promise, I'll be right beside you for as long as you need me, okay? We'll mourn Sans together.**

Frisk clung to him, even after he completely faded back to his non-corporeal form, even after Dipper and Mabel woke up and tactfully didn't ask why she was clinging to nothing, even when Toriel found them and took them home, putting them to bed so she could grieve herself. Papyrus came into her room and cried with them for a bit, but eventually retreated to his own room. It was three days before Frisk let go of Chara, and he stayed within arms' reach after that, ever-mindful of the random times she'd simply pull him back to her.

Things would never be the same, and never before had the desire to reset everything been this strong. It was laughable, really, that she only wanted to reset things months after the power to do so had been removed.

With time, they would heal. With determination, they would come back stronger. With love, they would forgive.

But for now, they would mourn.


End file.
